Petunias are native to Argentina, however; most of the petunias sold today are hybrids developed for specific purposes.
They are a popular bedding plant with wide trumpet shaped flowers in a multitude of colours and branching foliage that is hairy and somewhat sticky.
The flowers can be single or double, striped, veined or self coloured,and have a mounding or cascading habit, and sometimes they can be fragrant.
The most common varieties are the grandiflora and multiflora types,where grandiflora have the larger flowers, multiflora types hold up better during rainy weather.
A recent introduction is the Surfinia series.
These have become quite poular because they don’t need deadheading, and they can be used as bedding plants, groundcovers and or for trailing in containers.
All of this series are prolific bloomers and will bloom throughout the summer, particularly if they are regularly deadheaded to extend the flowering period.
Both types are popular plants for borders, window-boxes, tubs and hanging baskets, and are usually grown as half-hardy annuals.
The numerous garden varieties grow to a height of 200-400mm (8"-15") and flower from late June until first frosts, and prefer a sheltered sunny site and light well-drained soil.
Mixed Annual Petunias
Over rich soil, excessive shade and moisture will encourage leaf growth at the expense of flowers.
The delicate flowers can be damaged by wind and heavy rain, so purchase weather resistant varieties.
Sow seed on the surface of the seed compost and germinate at a temperature of 20°C (68°F).
Germination should take about a week.
Take tip cuttings from perennial varieties.
Prick out seedlings when large enough to handle, into trays of potting compost.
Alternatively you could purchase pre-germinated seeds from a garden centre.
Grow on for a couple of weeks at a minimum temperature of 12°(54°F), then gradually harden them off in a frost free coldframe until planting out time.
Pot up rooted cuttings into individual 70mm (3") pots.
Circa week 15 >
To encourage a branching habit pinch out the growing tips when the plants are 50-75mm (2"-3") tall.
Circa week 18:
Pinch out plant tips for a second time when the new shoots are 25mm (1") long.
This will delay flowering but will ensure an abundance of flowers.
As mentioned above deadhead regularly to prolong the flowering period.
When deadheading ensure that you remove the seed pod, similarly remove the seed pod if the petals have dropped off.
Plant out 250mm (10") apart when all fear of frost has passed.
Petunias perform best in full sun, but will tolerate partial shade, especially in hotter areas.
They will tolerate a range of soil pH and are drought resistant but don’t like to be dry for long periods, but equally, they don’t like it too wet!
Water regularly to avoid them drying out or becoming stressed, and feed every couple of weeks with a high potash fertiliser once they commence budding.
Extreme heat can cause petunia plants to stop setting flowers, however they will start again when the temperature drops.
Pests and diseases:
Petunias are generally trouble free except perhaps in persistant wet weather when the can contract grey mould or root rot.
Growing rain resistant varieties can alleviate this problem to a large extent.
Aphids can sometimes be a problem!
Spray with an insecticidal soap spray to reduce / clear the infestation.